CallbackManagerMixin

This method is called for non-chat models (regular text completion LLMs). If you're implementing a handler for a chat model, you should use on_chat_model_start instead.

This method is called for chat models. If you're implementing a handler for a non-chat model, you should use on_llm_start instead.

When overriding this method, the signature must include the two required positional arguments serialized and messages. Avoid using *args in your override — doing so causes an IndexError in the fallback path when the callback system converts messages to prompt strings for on_llm_start. Always declare the signature explicitly:

.. code-block:: python

def on_chat_model_start(
    self,
    serialized: dict[str, Any],
    messages: list[list[BaseMessage]],
    **kwargs: Any,
) -> None:
    raise NotImplementedError  # triggers fallback to on_llm_start